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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Exciton free induction in II-VI thin films observed by transient Brewster-angle reflectance spectroscopy

Takashi Kuroda, Kuon Inoue, Ikuo Suemune, Ryosuke Kuribayashi, F. Minami

Published Feb 9, 1995• 1 citation
Publisher

Abstract

The coherent emission process at excitonic resonance was first investigated in short-period ZnSe/ZnSxSe1-x (x equals 0.18) superlattices with one period of 50 A/50 A and 19 A/19 A by observing a temporal profile with subpicosecond time resolution, of reflected pulses in the Brewster-angle configuration. As a result, an almost background-free signal with nonresonant contribution reduced greatly, was observed reflecting free induction decay (FID) of coherently driven exciton polarization. In particular, a beat signal of FID was clearly observed for the first time, which arises from interference effect between a few split spectral lines of the relevant exciton. Further, it was confirmed that the decay spread of FID becomes faster as the power density of incident pulse was increased for the SL sample with a thicker well. The results may be explained qualitatively by a simple model with the knowledge of the exciton structure as well as a preliminary data of the phase relaxation time obtained by four- wave-mixing measurement. The present results have provided a basis for that the transient Brewster-angle reflection spectroscopy serves as a powerful tool for exploring coherent interaction of exciton with light.

DOI
10.1117/12.201449

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The coherent emission process at excitonic resonance was first investigated in short-period ZnSe/ZnSxSe1-x (x equals 0.18) superlattices with one period of 50 A/50 A and 19 A/19 A by observing a temporal profile with subpicosecond time resolution, of reflected pulses in the Brewster-angle configuration. As a result, an almost background-free signal with nonresonant contribution reduced greatly, was observed reflecting free induction decay (FID) of coherently driven exciton polarization. In particular, a beat signal of FID was clearly observed for the first time, which arises from interference effect between a few split spectral lines of the relevant exciton. Further, it was confirmed that the decay spread of FID becomes faster as the power density of incident pulse was increased for the SL sample with a thicker well. The results may be explained qualitatively by a simple model with the knowledge of the exciton structure as well as a preliminary data of the phase relaxation time obtained by four- wave-mixing measurement. The present results have provided a basis for that the transient Brewster-angle reflection spectroscopy serves as a powerful tool for exploring coherent interaction of exciton with light.
Coherent emission process of excitons in a layered compound GaSe， and in ZnSe/ZnSo．isSeo．s2 multiple quantum wells has been directly observed for the first time， by means of two independent newly-developed methods； time-resolved Brewster-angle reflectance （TRBR） spectroscopy and time-resolved four-wave-mixing （TRFWM） spectroscopy． In TRBR measurement， we have succeeded in observing a distinct free-induction-decay of coherently-driven polarization， with an oscillatory behavior due to finely split exciton levels． An analysis on the basis of a s imple model provides key information on the dynamical phase relaxation process． The result of the TRFWM measurement gives the comprehensive treatment of two dephasing processes； destructive interference due to static inhomogeneous broadening， and dynamical homogeneous phase relaxation． An analysis of TRFWM intensity enables one to exactly determine the homogeneous phase relaxation time． Finally， we show that a comparison between the coherent emission signal measured by TRBR or TRFWM， and third-order nonlinear correlation profile observed by time integrated FWM measurement enables one to make distinction between which of quantum - or classical interference effect causes the beat feature seen in the coherent emission signal．
The authors present an experimental analysis of the temporal structure of picosecond light pulses reflected from GaAs crystals near the lowest excitonic resonance at normal and Brewster angle incidence. The reflected pulses exhibit nonexponentially decaying tails, measured over five orders of magnitude, which follow the dynamics of the coherent polarization in the reflecting layer. Model calculations, including spatial dispersion effects and dead layer effects, provide a general understanding of the data. The numerical fit, however, fails to describe the fast initial decay observed experimentally. The latter can be explained by taking into account an inhomogeneous transition layer with a depth-dependent damping constant. Strong intensity dependence of the transients is observed.>
We have performed time-integrated as well as time-resolved femtosecond degenerate four-wave-mixing measurements on inhomogeneously broadened (Zn,Cd)Se/ZnSe quantum-well structures. Potential fluctuations due to alloy disorder in the ternary wells give rise to localized quasi-zero-dimensional excitons and biexcitons. For time-integrated detection, they manifest themselves by distinct (disorder-renormalized) quantum beats. Time resolving these transients reveals a free-induction-decay-like emission nature of the biexciton in contrast to the photon-echo exciton contribution. Using a five-level scheme, containing both biexcitons and antibound two-exciton states, this striking emission-type change is explained by fluctuation and correlation effects upon the biexciton binding energy. A combination of time-resolved and time-integrated four-wave mixing is thus a powerful tool to gain information about the disorder in a given kind of quantum structure.
The contribution of bound excitons and biexcitons to degenerate four-wave mixing signals is investigated in high-quality biaxially strained ZnSe layers. For the former complexes signatures of polarization interference with free excitons are found in time-resolved four-wave mixing, whereas spectrally resolved data point to the occurrence of quantum beating. Thus this system behaves as a mixture of independent two-level systems on the one hand and three-level systems on the other. Biexcitons contribute to the signal mainly at negative delay between the exciting pulses and are identified by means of spectrally resolved measurements and by use of different configurations of the pulse polarization. Complete agreement is found between the selection rules for biexcitons (and excitons) and the experimental data.
Spectrally resolved and time-integrated degenerate four-wave mixing (FWM) experiments with ultra-short pulses have been used to investigate the relaxation times and coherent interactions of excitons in the Boltzmann and in the quantum kinetic regime in ZnSe single quantum wells.Three-pulse FWM experiments using 100 fs pulses in a 10 nm thick ZnSe/ZnMgSe single quantum well reveal nearly equal zero-density dephasing rates for coherences between + σ and - σ heavy-and/or light-hole exciton states as compared to the dephasing rate of the interband coherence between these states and the ground state.The measurements also demonstrate that exciton-exciton scattering destroys the phase coherence between heavy-and light-hole excitons about three times more effectively than the coherence between + σ and + σ heavy-hole excitons.The different coherent nonlinear signals obtained in a three-beam FWM experiment are explained and reproduced by numerical calculations of the optical Bloch equations for a ten-level model including excitation induced dephasing (EID).Two-beam FWM experiments using 30 fs pulses on a 10 nm ZnSe quantum well reveal marked quantum beats and a pronounced non-exponential decay for delay times smaller ~500 fs.The pronounced peak at pulse overlap is attributed to rapid destructive interference of the created polarization and to spin dependent quantum kinetic effects caused by the transient formation of an electron-hole (eh) pair grating.The three-beam FWM trace shows a bi-exponential decay which is attributed to a nonlinear polarization that is caused by the interaction of polarization 3 k with both an exciton grating and an eh-pair grating.Model calculations using the OBEs of a two level system that includes ∑ -+
Abstract This paper studies coherent excitons in the presence of alloy disorder by femtosecond degenerate four‐wave mixing on ternary (ZnCd)Se/ZnSe quantum wells. The use of very low excitation densities enables to uncover an impurity‐mediated exciton dephasing time of (16 ± 2) ps and a related homogeneous exciton width of only 80 μeV. At higher densities, the coherence loss speeds up dramatically due to exciton‐exciton interaction. Separating the contributions from inhomogeneous (photon echo) and homogeneous broadening to the spectrally resolved decay transients allows to deduce the energy dependence of the dephasing time across the disorder broadened exciton ensemble. Further, pronounced biexciton resonances are observed in the signal spectrum which exhibit characteristic polarization properties. For cross‐polarized pulses, the biexciton signal shows a striking stimulation when the excitation density is increased.